The marginal band (MB) is a continuous peripheral bundle of microtubules, originally discovered in the elliptical, nucleated erythrocytes of non-mammalian vertebrates. It is known to be present as well in mammalian blood platelets, primitive mammalian erythrocytes (yolk-sac), invertebrate coelomocytes, and during erythrogenesis in mammals of the camel family. Its probable role in generation and/or maintenance of cell morphology is presently not well understood. Lysis of nucleated erythrocytes (non-mammalian) under microtubule-stabilizing conditions yields a "cytoskeleton" consisting of nucleus , MB, and an interconnecting network of trans-MB material (TMB). Our working hypothesis for generation of flattened discoid or elliptical cell morphology involves MB-TBM interaction (Cohen, J. C. B. 78, 1978). Current and proposed work includes "bulk" MB isolation and analysis by SDS-PAGE and TEM, small sample isolation of MBs from Amphiuma (largest known erythrocytes with thickest MBs) for SEM examination, continued studies of MB lability in vitro and in vivo (utilizing for the latter the coelomocytes of the lobster Homarus americanus which undergo drastic morphological changes), analysis of MB/TBM structure and composition by means of specific antibody immunofluorescence, and continued survey of new cell types and species for MB presence and properties.